Our recent work has demonstrated that the immunosuppressive drug rapamycin selectively affects the CsA-resistant pathway. Our initial studies have focused on the mechanism of activation of the IL-2 gene in a CsA-resistant manner. We found that the effect of rapamycin on the IL-2 expression was due to alteration in IL-2 mRNA stability. More recently, we have also shown that activation of T cells by IL-12 is resistant to CsA, but sensitive to rapamycin. As the intracellular target of rapamycin is mTOR (mammalian target of rapamycin), we are investigating the mechanism of activation of mTOR during T cell activation. Regarding the physiological role of the resistant pathway, we have observed the effect of cytokine signaling, particularly the combination of IL-12 and IL-18 but not individual cytokine alone, in activating resting human peripheral blood T cells in producing IFN-g in a CsA-resistant, but rapamycin-sensitive manner. Interestingly, nave CD4+ T cells are more responsive to IL-12 plus IL-18 stimulation in comparison to memory CD4+ T cells. This cytokine-mediated activation of resting T cells is independent of antigen. Intracellular staining reveals a small percentage of resting CD4+ T cells capable of producing IFN-g in response to IL-12 plus IL-18. We have shown that CD4+IL-18Ra+ cells are the main populations responsive to IL-12 plus IL-18. The expression of IFN-g induced by IL-12 and IL-18 is sensitive to rapamycin and SB203580, indicating the possible involvement of mTOR and p38 MAP kinase, respectively, in this synergistic pathway. We have also shown that IL-12 plus IL-18-induced IFN-g mRNA expression is independent of new protein synthesis and independent of GADD45b and g. We are currently investigating the in vivo physiological role of this cytokine signaling pathway.